This is mostly my opinion, but:
They look great, especially the nexus killing animation and the fountain layout. The poro eating biscuits? Goddamn lovely. It's on point.
But that's the problem. Individually the new art assets are ridiculously cool, but they just look bad on the brown/green background of the Summoner's Rift, and ultimately look bland.
Maybe having the new assets look darker would fix it, or changing the colour of the rift. I don't know. It's just my two cents. The new assets, while amazing on their own, look dreadful on the rift